1. Field of the Invention
The present invention relates to optical substrates having a structured surface, particularly to optical substrates for brightness enhancement, and more particularly to brightness enhancement substrates for use in flat panel displays having a planar light source.
2. Description of Related Art
Flat panel display technology is commonly used in television displays, computer displays, and handheld electronics (e.g., cellular phones, personal digital assistants (PDAs), etc.). Liquid crystal display (LCD) is a type of flat panel display, which deploys a liquid crystal (LC) module having an array of pixels to render an image. In backlight LCDs, brightness enhancement films use prismatic structures to direct light along the viewing axes (i.e., normal to the display), which enhances the brightness of the light viewed by the user of the display and which allows the system to use less power to create a desired level of on-axis illumination.
Heretofore, brightness enhancement films were provided with parallel prismatic grooves, lenticular lenses or pyramids on the light emitting surface of the films, which change the angle of the film/air interface for light rays exiting the films and cause light incident obliquely at the other surface of the films to be redistributed in a direction more normal to the exit surface of the films. The brightness enhancement films have a light input surface that is smooth, through which light enters from the backlight module.
Heretofore brightness enhancement films are made up of two layers, including a support base layer and a structured layer. FIG. 1 depicts a sectional structure representative of prior art brightness enhancement films. The brightness enhancement film 100 includes a base layer 102 made of polyethylene terephthalate (PET), and a structured layer 104 of prism structures made of acrylic, which function to redirecting light. The adhesion between these two layers is important to the integrity, reliability and performance of the brightness enhancement film 100. The integrity of the adhesion is affected by environmental conditions, such as moisture and heat. For example, when the brightness enhancement film 100 is subject to moisture and high temperature, the structured layer 104 could separate from the base layer 102, which separation between the layer would result in artifacts in the display image, such as undesirable bright lines in the display image.
The structured surface of brightness enhancement film 100 is formed after bonding a layer of materials (e.g., an acrylic layer) to the base layer 102 prior to forming the prism structures in the acrylic layer to form the structured layer 104. The prism structures in the structured layer 104 may be formed using a number of process techniques, including micromachining using hard tools to form master molds or the like for the prism structures. The hard tools may be very small diamond tools mounted on CNC (Computer Numeric Control) machines (e.g. turning, milling and ruling/shaping machines), such as known STS (Slow Tool Servo) and FTS (Fast Tool Servo). U.S. Pat. No. 6,581,286, for instance, discloses one of the applications of the FTS for making grooves on an optical film by using a thread cutting method. The tool is mounted onto the machine, to create longitudinal prisms in a plane. The mold may be used to form the structured layer through hot embossing a substrate, and/or through the addition of an ultraviolet curing or thermal setting materials in which the structures are formed.
As shown in FIG. 1, the bottom of the valleys 106 of the prisms in the structured layer 104 is not at the surface of the base layer, but spaced at a distance d from the contacting surface of the base layer by acrylic material. It has been found that the bottom thickness d plays an important role in maintaining integrity of the adhesion. In the past, the bottom thickness d range between 0.3 to 3 micrometers. In order to obtain the bottom thickness, several parameters must be controlled, e.g., during the curing process to form the structured surface. For example, nip roll pressure, acrylic resin temperature, line speed, etc. are some of the factors to be taken into consideration. However, it is challenging to control a consistent bottom thickness d. Alignment errors could be introduced by the mechanical system used to form the structured surface. For example, dynamic mechanical alignment errors from the master roll and curing machine are on an order comparable or larger than the bottom thickness d. As a result, those factors would introduce some unwanted cosmetic defects such as ‘chatter’ and/or non-uniformity of the brightness enhancement film. The results in a phenomenon that is easily seen for existing brightness enhancement films, in which repeated dark shades/lines are seen from light from the planar light source transmitted through the brightness enhancement film.
What is needed is an optical substrate that provides a surface structure that both enhances brightness and reduces chatter phenomenon.